Technical Field
The present invention relates to a communication apparatus, signal processing method, and a non-transitory recording medium storing a signal processing program.
Background Art
Many wired communication networks have come to be replaced by networks. In particular, with the release of apparatuses that have only a wireless communications interface, such as tablet devices and smartphones, it is projected that wireless communication networks will continue to be increasingly used. Accordingly, the speed of wireless communication networks has increased spectacularly in response to increasing demand. As a result, it has become possible to transfer large-volume data such as still images and video and real-time data such as audio wirelessly.
Wireless communication circuits in terminal devices incorporated in wireless communication systems (wireless transmitters and wireless receivers) perform communication in synchronization with a predetermined series of time slots. Furthermore, when transferring data including both video and audio signals in synchronized time slots wirelessly, it is necessary to synchronize an encoder in the wireless transmitter with a decoder in the wireless receiver in order to synchronize the video signal with the audio signal.
MPEG-2 Transport Stream (TS) technology is one widely used technique for synchronizing the video signal with the audio signal and is used in fields such as digital broadcasting to compress TS packets that include the video signal and the audio signal using MPEG-2 and H.264 formats. In addition to the video signal and the audio signal, various other data is stored in the TS packets, including a Program Clock Reference (PCR) value. The PCR value indicates current time information of the encoder. The decoder controls output timing of the video signal and audio signal by adjusting an internal timer value using the PCR value and synchronizes the video signal with the audio signal.
By adopting MPEG-2-TS in the wireless communication system, it is possible to perform encoding and decoding of data in real time. However, in order to implement high-quality encoding and decoding in real time, it is necessary to minimize packet dropout and absorb fluctuations in propagation time because, in some cases the quality of the video signal and audio signal cannot be maintained.
In MPEG-2-TS, as described above, the decoder absorbs any skew in the clock signal to track the time information of the encoder using the PCR value, and the decoder is synchronized with the encoder. However, if the propagation time of the TS packet that stores the PCR value varies, its arrival time also varies, and the time information of the encoder acquired from the PCR value varies as well. In this case, since the decoder follows the varied time, oscillating frequency of the clock signal goes up and down substantially, resulting in distorting the video signal and causing color shift. To cope with this issue, the MPEG-2-TS specifications stipulate that PCR tolerance is to be kept to within several tens of μsec.
Generally, the size of the wireless transferred frame is different from the size of the TS packet. While the size of the TS packet is 188 bytes or 204 bytes, the maximum frame size for wireless LAN defined in IEEE 802.11a/b/g is 2304 bytes. In IEEE 802.11n, aggregation and other techniques are used for enhancing wireless speed, and the maximum frame size is 65535 bytes.
In MPEG-2-TS, a null frame is used as one type of TS packet if there is no TS packet transferred for adjusting bit rate, etc. Since this null frame is meaningless information that has no relation to wireless transferred media information and the network bandwidth is squeezed if lots of null frames are included, it is ideal that no null frame is included.
However, when not using the null frame, if the TS packet that includes the PCR value is generated and stored in the wireless frame, in some cases, the predetermined frame size is not reached, and there is no subsequent TS packet for a certain period of time. In this case, the TS packet that includes the PCR value cannot be wireless transferred. As a result, the propagation time in wireless transmission fluctuates substantially. For example, a case in which the boundary of TS packetized video ES data does not correspond to the boundary of the size of the wireless frame is one of those cases.